fix: Change auth blueprint prefix from /admin to /auth (v0.9.2)

The auth routes were registered under /admin/* but the IndieAuth
redirect_uri was configured as /auth/callback, causing 404 errors
when providers redirected back after authentication.

- Change auth blueprint url_prefix from "/admin" to "/auth"
- Update test expectations for new auth route paths
- Add ADR-022 documenting the architectural decision

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

tests/test_routes_admin.py

@@ -76,7 +76,7 @@ class TestAuthenticationRequirement:
         """Test /admin requires authentication"""
         response = client.get("/admin/", follow_redirects=False)
         assert response.status_code == 302
-        assert "/admin/login" in response.location
+        assert "/auth/login" in response.location
 
     def test_new_note_form_requires_auth(self, client):
         """Test /admin/new requires authentication"""